At first I thought this might be a Firefox issue, but I get the same problem in Chrome, so it seems to be more system wide.
When I try to play any online video, particularly YouTube, the audio plays fine, but the video holds on one frame indefinitely. Sometimes it suddenly speeds up to catch up with the audio, but then freezes again. It is, in short, unwatchable.
I tried the solution offered here, but with the gnash player installed, online videos will not play.
This was not a problem before, and I think something has gone wrong with a recent upgrade.
I have the non-free Flash player installed, as well as ubuntu-restricted-extras.
How can I get video to play again?